Study of structure, phase transformations, magnetocaloric effect, and critical behavior in Ni51-xFexMn28Ga21 magnetic shape memory alloys

This study investigates the magnetostructural properties, magnetocaloric effect (MCE), and critical behavior of Ni 51 -x Fe x Mn 28 Ga 21 ( x = 0, 1, 2, and 3 at.%) magnetic shape memory alloys. Room-temperature X-ray diffraction reveals that the x = 0 and x = 1 compositions crystallize in a seven-layered orthorhombic martensitic structure. In contrast, the x = 2 and x = 3 alloys adopt a five-layered modulated tetragonal structure. Fe substitution causes moderate composition-dependent changes in the lattice parameters and shifts the martensitic transformation (MT) toward room temperature. These changes correlate with variations in the electron-to-atom ratio, local structural/compositional disorder, and unit-cell volume. Fe substitution also leads to reduced antiferromagnetic interactions, stabilization of the martensitic phase, pronounced magnetization jumps, and a slight broadening of the MT hysteresis. Among the Fe-doped alloys, the x = 2 composition exhibits the most favorable low-field MCE at 1.5 T. Critical-behavior analysis based on Arrott plots, modified Arrott analysis, the Kouvel-Fisher method, and the critical isotherm indicates predominantly second-order magnetic phase transitions for the x = 2 and x = 3 alloys. The corresponding effective critical exponents remain broadly close to mean-field values. • Fe doping changes the martensitic structure in Ni 51 -x Fe x Mn 28 Ga 21 alloys from 7 M ( x = 0, 1) to 5 M ( x = 2, 3). • Fe doping shifts the martensitic transformation (MT) temperature toward room temperature. • Fe doping modifies the magnetization jump, while the x = 2 alloy exhibits the most favorable low-field magnetocaloric response at the MT among the Fe-doped alloys. • Critical-behavior analysis indicates effective critical exponents broadly close to mean-field values for the x = 2 and x = 3 alloys.